Until the discovery of chiral activity, the development of electromagnetic radiation absorbing material was limited to design variations in .epsilon., the permittivity, and .mu., the magnetic permeability of the absorbing material. In order to match the impedance of the absorbing material to the free space impedance .epsilon..sub.r (relative permittivity) and .mu..sub.r (relative permeability) are ideally equal. Practical consideration, however, dictate that this is not possible, with the result that the development of absorbers had been confined to searching for compromise combinations of .epsilon..sub.r and .mu..sub.r for the absorbing material which produce the best results. There were therefore 2 degrees of freedom in choosing the properties of the absorber.
The introduction of chirality into an absorber produces an extra degree of freedom, expressed by the chirality parameter .beta., for the choice of properties of the absorbing material.
Chirality is the handedness of an object, that is, the property of an object which renders it non-congruent with its mirror image. Work carried out on chiral absorbing materials indicates that electromagnetic radiation incident on an absorbing material containing chiral inclusions is caused to decompose into left-and right-circulatory polarized forms and be scattered through the lossy dielectric material which is host to the chiral inclusions.
According to the present invention there is provided an electromagnetic radiation absorbing structure comprising a structure for absorbing radiation in the range 10 megahertz to 100 gigahertz comprising: (a) a geometrically shaped lossy dielectric medium; and, (b) chiral elements disposed outside the lossy dielectric medium.
The teaching of the prior art is that chirality is only effective when, in a lossy host. Tests conducted on chiral elements alone show no absorptive properties. With the structure of the present invention, it has been found that enhanced absorption can be achieved by the addition of chiral elements to the exterior surface of a lossy dielectric material, that is, the chiral elements do not have to be embedded within a lossy dielectric host, but rather may be adjacent it while still enhancing the absorption properties of the overall structure.